Many clinical conditions, deficiencies, and disease states can be remedied or alleviated by supplying to the patient one or more biologically active molecules produced by living cells or by removing from the patient deleterious factors which are metabolized by living cells. In many cases, these molecules can restore or compensate for the impairment or loss of organ or tissue function. Accordingly, many investigators have attempted to reconstitute organ or tissue function by transplanting whole organs, organ tissue, and/or cells, which provide secreted products or affect metabolic functions. However, while transplantation can provide dramatic benefits, it is limited in its application by the relatively small number of organs that are suitable and available for grafting. In general, transplantation patients must be immunosuppressed in order to avert immunological rejection of the transplant, which results in loss of transplant function and eventual necrosis of the transplanted tissue or cells. Likewise, in many cases, the transplant must remain functional for a long period of time, even for the remainder of the patient's lifetime. It is both undesirable and expensive to maintain a patient in an immunosuppressed state for a substantial period of time.
A number of vision-threatening disorders of the eye exist for which additional good therapies are still needed. One major problem in treatment of such diseases is the inability to deliver therapeutic agents into the eye and to maintain them there at therapeutically effective concentrations.
Many growth factors have shown promise in the treatment of ocular disease. For example, BDNF and CNTF have been shown to slow degeneration of retinal ganglion cells and decrease degeneration of photoreceptors in various animal models. See, e.g., Genetic Technology News, vol. 13, no. 1 (January 1993). Additionally, nerve growth factor has been shown to enhance retinal ganglion cell survival after optic nerve section and has also been shown to promote recovery of retinal neurons after ischemia. See, e.g., Siliprandi, et al., Invest. Ophthalmol. & Vis. Sci., 34, pp. 3232-3245 (1993).
A desirable alternative to transplantation procedures is the implantation of cells or tissues within a physical barrier which will allow diffusion of nutrients, metabolites, and secreted products, but will block the cellular and molecular effectors of immunological rejection. A variety of macrocapsule devices which protect tissues or cells producing a selected product from the immune system have been explored. See, e.g., U.S. Pat. No. 5,158,881; WO92/03327; WO91/00119; and WO93/00128, each of which is incorporated herein by reference in its entirety. These devices include, for example, extravascular diffusion chambers, intravascular diffusion chambers, intravascular ultrafiltration chambers, and implantation of microencapsulated cells. See Scharp, D. W., et al., World J. Surg., 8, pp. 221-9 (1984). See, e.g., Lim et al., Science 210: 908-910 (1980); Sun, A. M., Methods in Enzymology 137: 575-579 (1988); WO 93/03901; and U.S. Pat. No. 5,002,661. Such devices would alleviate the need to maintain the patient in an immunosuppressed state. However, none of these approaches have been satisfactory for providing long-term transplant function.
Thus, methods of delivering appropriate quantities of needed substances, such as, neurotrophic factors, anti-angiogenic factors, anti-inflammatory factors, enzymes, hormones, other factors or, of providing other needed metabolic functions, to the eye for an extended period of time are needed.